In order to secure electrical fixtures such as lights and fans in ceilings, a hanger bar device is typically employed. It is very important that a fixture be adequately supported in the ceiling since improper support could lead to the fixture breaking away from the ceiling. Ceilings are typically constructed of structural members including a series of spaced joists which are covered by lathe and plaster or a gypsum wallboard often referred to as drywall. The ceiling covering material or sheathing is not designed to support loads perpendicular to its surfaces. Therefore, in order to suspend electrical fixtures from a ceiling, the fixture's weight must be supported by the joists. To achieve this, a fixture may be secured directly to a joist, however, this limits the range of locations where the fixture may be placed. Such a limitation is problematic since it is often desirable to place the fixture in a specific position on the ceiling. Therefore, if no joist is adjacently positioned to the desired location, direct joist connection is not possible. A preferred solution for supporting electrical fixtures in the ceiling is the use of a hanger bar assembly. The hanger bar allows for the joists to be used for supporting an electrical fixture while still providing flexibility in positioning.
Hanger bar assemblies are typically extensible members that are disposed in between the ceiling joists and are securable thereto. An assembly's length, therefore, is adjustable to accommodate various joist spacing distances. Electrical outlet boxes, junction boxes and the like are then secured to the transverse members of the assembly. Such electrical boxes provide a termination point for electrical wiring as well as an attachment point for the electrical fixture. Typically the outlet box can be positioned at various points along the length of the members in order to properly position the fixture attached thereto. In conventional construction a cut out is made in the ceiling covering material in order to provide clearance for the electrical outlet box, thereby allowing the fixture to be secured to the outlet box.
The outlet box once secured to the hanger bar assembly is typically used to support the electrical fixture. The National Electric Code sets a maximum weight that an outlet box may support. Outlet boxes may either support lighting fixtures weighing up to 50 lbs. or ceiling fans weighing up to 35 lbs. Fixtures exceeding these weight limits must be supported independently of the box such as directly to the joists or other structural member.
Hanger bar assemblies can be divided into two general categories, standard and heavy duty and are distinguishable by the amount of weight they can safely support. Standard hanger bars are typically used to support lightweight lighting fixtures that are generally less than five pounds.
U.S. Pat. No. 2,925,236 to Cook et al. discloses a standard duty hanger bar assembly. The assembly includes telescopically engagable slotted channel members positionable between two ceiling joists. A box carrier is inserted in the channel members and has guide prongs extending from the slots. An outlet box is insertable over the prongs and is secured to the assembly by a box fastener which is securable to the box carrier by a threaded bolt. Upon the tightening of the bolt, a portion of the outlet box and a portion of the channel members are clamped together between the box carrier and fastener, thereby securing the box to the assembly. The box carrier, however, only engages a small amount of channel section thereby providing a relatively small clamping area. Accordingly, this assembly is suited for light duty applications such as small to medium sized light fixtures, which present static loadings.
Heavy duty hanger bars are typically employed for supporting heavy lighting fixtures up to 50 lbs. and ceiling fans weighing 35 lbs. Ceiling fans require high load bearing supports not only due to their weight but also due to the torque and vibrational forces that are generate by the rotation of the motor and fan blades. Relatively small weight differences between the blades of a ceiling fan can produce rotational imbalances leading to large vibrational forces. Therefore, a heavy duty hanger bar is required when suspending electrical fixtures such as ceiling fans.
Heavy duty hanger bars assemblies must be able to maintain their rigidity and length under high load situations. A load acting perpendicular to the length of the telescopic members tends to deflect the assembly toward the direction of the load. The degree of deflection increases when the members are free to move relative to each other as the members tend to be pulled apart by the load. Under certain circumstances, the deflection could be to such a degree that the members could be pulled completely apart resulting in the release of the outlet box and fixture attached thereto. Therefore, it is important that once the assembly is installed the members be secured so that they resist motion between themselves.
Heavy duty hanger bars may be designed for installation in new work or old work applications. New work applications include installations where the ceiling covering material has not yet been installed and access to the joists is available. This may occur in situations such as new construction or renovations where the ceiling material has been removed. Old work applications include installations where the ceiling covering material is already installed and the hanger bar needs to be inserted into an outlet box clearance opening cut into the ceiling. In such applications access to the joists is typically extremely limited. This situation is typically found when a ceiling fan is to be installed in a location where a ceiling lighting fixture had previously been installed.
A hanger bar assembly capable of supporting loads such as ceiling fans and adapted for use in new work applications has been disclosed in the Heavy Duty Slide Support System by Reiker Enterprises Inc, U.S. Pat. No. 5,938,157. This device includes telescopically engaged slotted channel members. The channels have a bottom wall formed by inwardly turned flanges defining a slot. Disposed within the channel members is a substantially planar plate having downwardly curved edges. The plate has two threaded bolts extending downwardly through the aligned slots. The threaded bolts extend through clearance holes in an outlet box and provide points on which to secure a fixture. A third threaded bolt extends from the center of the plate and is used to secure the box to the plate. The flanges of both channels extend downwardly at a slight angle. When the outlet box is secured to the channel members the flanges are forced together thereby clamping the two channel members together. The flanges, however, are relatively short thereby creating only a small amount of contact area between the flanges. Such limited amount of contact or clamping area limits the assembly's ability to resist slippage.
The Reiker Heavy Duty Slide Support System further includes a pair of end plates extending from the opposed ends of the assembly. The end plates are formed from upwardly bent extensions of the top wall of the channels. The end plates, therefore, are connected to the channels by a relatively thin strip of material. This design limits the assembly's ability to resist vibrational forces acting perpendicular to the channels. Such forces tend to cause the channel members to flex about the connection strip. This problem is exasperated by locating the connection strip above the assembly is neutral axis passing through the longitudinal centroid of the channel members. The offset between the connection strip and the neural axis creates a moment about the thin strip of connecting material and further reduces the assembly's ability to resist vibrational forces.
U.S. Pat. No. 5,934,631 to Becker et al. further discloses a telescoping hanger bar assembly that is capable of supporting relatively heavy loads such as ceiling fans. The channel members each have a different profile to allow one to slide freely within the other. A plate is supported within the channel members and is attachable to an outlet box, which is rated to support ceiling fans. One such outlet box is described in U.S. Pat. No. 5,824,952. Attachment of the box to the plate results in a clamping force which urges overlying flanges of the channel members into engagement.
The telescoping members of the prior art hanger bars are typically channels formed having different cross-section so that one member is received in the other member. In order to achieve rigidity between the members, it is important to obtain a close fit between the two members. However, the fit must permit the members to freely slide so the length of the hanger bar can be easily adjusted to accommodate securement between the joists. This is very difficult to consistently achieve especially when the members are extruded. Extruded profiles will vary from run to run based on the thermal expansion of the die. Since the channel members must always slidingly fit together, the members are necessary toleranced to permit a certain amount of play between the members. This play between members, however, reduced the rigidity of the assembly and limits its load bearing and vibration resistance capability.
Accordingly, it would be desirable to devise a hanger bar assembly capable of supporting heavy loads such as ceiling fans that can be quickly installed and relatively simple to produce as well as being sufficiently rigid to resist vibrational forces under the heavy load of a fixture.